Classic Shell Scripting

Preface

The user or programmer new to Unix^[1] is suddenly faced with a bewildering variety of programs, each of which often has multiple options. Questions such as "What purpose do they serve?" and "How do I use them?" spring to mind.

This book's job is to answer those questions. It teaches you how to combine the Unix tools, together with the standard shell, to get your job done. This is the art of shell scripting. Shell scripting requires not just a knowledge of the shell language, but also a knowledge of the individual Unix programs: why each one is there, and how to use them by themselves and in combination with the other programs.

Why should you learn shell scripting? Because often, medium-size to large problems can be decomposed into smaller pieces, each of which is amenable to being solved with one of the Unix tools. A shell script, when done well, can often solve a problem in a mere fraction of the time it would take to solve the same problem using a conventional programming language such as C or C++. It is also possible to make shell scripts portable—i.e., usable across a range of Unix and POSIX-compliant systems, with little or no modification.

When talking about Unix programs, we use the term tools deliberately. The Unix toolbox approach to problem solving has long been known as the "Software Tools" philosophy.^[2]

A long-standing analogy summarizes this approach to problem solving. A Swiss Army knife is a useful thing to carry around in one's pocket. It has several blades, ...